Conventionally, for an inkjet recording apparatus, as a colorant, a hydrophilic ink comprising a water-soluble dye is mainly used. The dye ink has a disadvantage to be poor in weathering properties and water resistance. Therefore recently, studies on a pigment ink produced using a pigment instead of the water-soluble dye have been developed. However, the pigment ink is poor yet in color developing properties and stability in comparison with the dye ink and particularly accompanying with the improvement of the technique for the high quality image of the printer for the OA, with respect to the pigment ink used for the plain paper, plain paper properties, such as printing quality, hue, color saturation, gloss and shelf stability compared to those of the dye ink are required.
As a material for a magenta ink or a cyan ink which are used as a pigment ink, respectively C.I. pigment red 122 or pigment blue 15:3 is generally used; however, the color reproduction range of these pigment inks differs from that of the dye ink. Therefore, the toning for minimizing the hue error is performed and as a result of the toning, the color saturation is lowered and a disadvantage is caused with respect to the printing quality.
On the other hand, the improvement of the pigment itself for change the hue without the toning has been developed, for example, a cyan pigment having a hue in the same color gamut as that of the cyan dye produced using a phthalocyanine pigment having a specified crystal structure is proposed (see Japanese Patent Application Laid-Open (JP-A) No. 2000-17207). However in this case, all problems, such as a problem with respect to the cost, are not yet solved.
Further, many proposals are performed with respect to an ink set comprising a black ink produced using a pigment, a yellow ink, a magenta ink and a cyan ink which are produced using a dye (see JP-A No. 2000-239590). However, in these proposals, the inks are unsatisfactory in plain paper properties when the plain paper is printed using the inks.
An ink set consisting of a black ink comprising a self-dispersible carbon black and color inks comprising a colorant having a reverse polarity to that of the colorant of the black ink is proposed (see JP-A No. 10-140064). In addition, an ink set comprising inks of a colorant-impregnating resin dispersion type which have various ionicities each other is proposed (see JP-A No. 2000-191972). However, in these proposals, with respect to the print produced using the ink set, spreading of the ink at the interface of two colors can be improved; however, other plain paper properties were unsatisfactory yet.
Further, it is proposed that as a water-soluble organic solvent in a hydrophilic ink composition, for improving the moisture retention of the pen head in the case where the ink is used for a writing material, 3-methyl-1,3-butanediol is used (see JP-A No. 04-106168). However, in this patent document, the improvement of plain paper properties as a recording ink is neither disclosed nor indicated.